How would or could a President Obama ban concealed carry both legally and politically?

By executive order, a president could conceivably ban concealed weapons on all federal property. Such an EO would run afoul of several rule-making provisions already in place, which would open the EO to a challenge in court.

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What would be the legal and political mechanism and fallout from an attempt to ban concealed carry in every state.

As stated above, one legal mechanism would be an EO, although it would affect only federal property. Another mechanism would be to get the legislature to pass a nationwide ban.

The fallout would be almost immediate for such a law. The precedent of State issued drivers licenses would be in question, should such a law not be challenged. The States will not let that happen. The Congress, as recently as 2007, could not subsume lawful State police powers with the Real ID Act, nor can it in this case (the 10th amendment question).

Considering that State laws on concealed carry are predicated upon personal self defense, there would also be a challengeable action under Heller, assuming arguendo that incorporation has already occurred. If not, then the incorporation issue would be raised in every Circuit, and where such incorporation occurs, the law would then be challenged under Heller.

To sum it up, there are two immediate actionable causes that I see. 1) Such a law infringes the police power of the individual states (10th amendment) and 2) such a law infringes the 2A itself, given that the State allows such, even if via licensing.